A variety of antenna systems have been developed for use in receiving and transmitting electromagnetic energy. In that regard, most antenna systems are designed for use with a single portion of the electromagnetic spectrum, falling somewhere between the low-frequency audio end of the spectrum and the high-frequency ultraviolet end of the spectrum.
Some antenna systems have, however, been developed for use with more than one portion of the electromagnetic spectrum. Multiple-mode antenna systems are used, for example, in military and industrial detection systems to receive radiation from moving and stationary objects within the antenna system's field of view, allowing the objects to be accurately detected, analyzed, and tracked by the detection system. In such systems, any shortcomings attributable to operation of the detection system over one portion of the electromagnetic spectrum may be compensated for by operation of the system over the other portion of the spectrum.
Of particular interest are dual-mode antenna systems operable, for example, in the radio frequency (RF) and electro-optical (EO) portions of the electromagnetic spectrum. The RF spectrum includes millimeter wave (MMW), super high frequency (SHF), and ultrahigh frequency (UHF) radiation. Systems operating in this frequency range can detect objects over long distances and wide areas, with relatively little interference from environmental elements such as rain, fog, atmospheric particulate, and haze. Due to the wide beamwidth employed, however, RF systems typically exhibit relatively low angular resolution.
The EO spectrum is much higher in frequency than the RF spectrum and includes ultraviolet (UV), visible, and infrared (IR) radiation. Because EO systems employ relatively narrow beamwidths, they are able to resolve objects with high angular resolution. Unfortunately, the range of EO systems is limited in adverse conditions.
A detection system including a dual-mode antenna system for receiving both MMW and IR radiation advantageously allows objects to be selectively detected over long ranges, with a high degree of accuracy and a limited amount of interference from environmental elements. In a dual-mode system, detectors responsive to the received MMW and IR radiation produce detection outputs that are correlated and alternatively relied upon to provide the desired performance characteristics.
The development of a suitable dual-mode MMW/IR antenna system has not, however, been easily achieved. As will be appreciated, conventional MMW and IR systems operate differently and employ different components and materials. In fact, many of the design constraints applicable to the two types of systems are mutually exclusive.
By way of illustration, sensors responsive to radiation in the MMW portion of the spectrum typically will not respond to IR radiation and sensors responsive to IR radiation typically will not respond to MMW radiation. As a result, separate detectors are required for the two portions of the spectrum. The detectors must, however, be coaxially aligned if their outputs are to be correlated. The mechanical integration necessary to achieve coaxial alignment introduces significant complications and interferences into the design.
One example of a prior art, dual-mode, MMW/IR antenna is provided in U.S. Pat. No. 4,636,797 (Saffold et al.). The Saffold et al. antenna includes a Cassegrain optics section that coaxially collects incident MMW and IR radiation. More particularly, radiation impinging upon a primary reflector of the optics section is reflected to a secondary reflector, where it is again reflected to an energy-collecting radome. The radome passes both MMW and IR radiation to a waveguide assembly that is coupled to the optics section.
A dichroic beam splitter is included in the waveguide assembly to separate the MMW and IR beams for subsequent processing. Specifically, the IR beam is reflected 90 degrees by the beam splitter before being redirected by another reflector to lenses and an IR processing section. The MMW beam, on the other hand, is transmitted without interruption by the beam splitter to a lens and a MMW quasi-optic processing section.
While a dual-mode antenna constructed in this manner does work, it has several shortcomings. First, the transmission of both beams through the radome, along with the transmission of the MMW beam and reflection of the IR beam at the dichroic splitter, may introduce significant losses into the system. Although not addressed in the patent, these losses are further exacerbated by the requiste addition of an external radome to the antenna for most applications. Second, the separation of the two beams and their eventual transmission along individual paths requires a relatively complex mechanical design.
Another example of a prior art, dual-mode, MMW/IR antenna is provided in U.S. Pat. No. 4,652,885 (Saffold et al.). This antenna, like that described in the '797 patent, employs a Cassegrain optics section to collect incident MMW and IR radiation. Upon reaching the waveguide, the MMW radiation is focused by a lens into the MMW quasi-optic processing circuit. The IR radiation, on the other hand, appears to be processed in one of several different ways.
For example, in the arrangement shown in FIG. 1 of the patent, a plurality of IR optical fibers are attached to the back of the MMW focus lens and conduct at least some of the IR beam to an IR processing section. Although the ends of the IR optical fibers do not appear to coincide with a focal point of the IR beam, the Cassegrain optics would not really provide MMW and IR beams in parallel as shown in FIG. 1. In another arrangement, shown in FIG. 3 of the '885 patent, the ends of the IR optical fibers are positioned behind the MMW focus lens, at the focal point of the IR beam and forward of the focal point of the MMW beam.
A dual-mode system constructed in the manner taught by the '885 Saffold et al. patent has several disadvantages. In that regard, because the IR optical fibers are located behind the MMW focus lens, the lens must be able to transmit both the MMW and IR beams. While such lenses exist, they must include an impedance matching layer for both the IR and MMW radiation. Without this layer, the IR and MMW radiation will be reflected at the lens surfaces. Such reflections, inevitably introduce greater losses into the system.
Along with the shortcomings attributable to location of the IR optical fibers at the back of the MMW focus lens, the Saffold et al. systems have several additional disadvantages. For example, the inclusion of the internal radome at the end of the waveguide introduces losses but does not eliminate the need for an external radome covering the Cassegrain optics section. Thus, although two radomes are not shown in the Saffold et al. arrangements illustrated in the patents, they would, in practice, be employed, contributing to system losses.
Also, the Saffold et al. designs are intended for single-point detection. As will be appreciated, however, it is frequently desirable to detect images. Although multidimensional detection can be accomplished by manipulation of a single-point detector, the drive requirements for such a system would be rather complex and expensive.
In view of the preceding observations, it would be desirable to provide an antenna system that can be used in multiple modes, without exhibiting large losses, and that can be used for image detection.